Set dimensions of a mesh
That's correct.

The scale is an arbitrary number which is used to offset the size of the mesh from it's original state. It doesn't refer to the actual size of the mesh, just how much bigger or smaller it is.

I think you are looking for the dimensions values in the properties region:



Setting the dimensions will set the scale values to the appropriate value to make the mesh a certain size, e.g. 2.5m tall: